Monitoring metal ion leaching in oil-ZnO paint systems with a paramagnetic probe

Abstract

Zinc oxide (ZnO), used as a pigment since the 19 th C., is highly reactive when mixed with drying oils. Indeed, the combination of metal-based pigments and drying oils may react to form metal complexes in paint, which may lead to the aggregation of metal carboxylates or soaps. Whereas the mechanism and chemistry behind metal soap formation has been studied in depth, little research has focused on the changes that affect the inorganic pigment particles. In this work, we concentrate on the first phase of these reactions and monitor the evolution of an oil-ZnO paint system through Electron Paramagnetic Resonance (EPR) spectroscopy. By employing Cu II as a paramagnetic probe, the progression of metal ion leaching from ZnO is followed through the switching of Cu II from a silent tetrahedral coordination in ZnO bulk to an EPR active pseudo-octahedral coordination. Complementarily, the effective concentration of dissolved or extracted Zn and Cu is quantified through Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The experiment probes the spontaneous metal ion release in oil-based paint films and demonstrates that the kinetics of this phenomenon proceeds with a very steep increase in the first hours after mixing. This is followed by the saturation of the grow rate after a few days that we ascribe to the passivation of the ZnO pigment surface by carboxylate groups, which hinders the further leaching of metal ions, leading to a steady-state before the complete hardening or oxidation of the paint film.

Experimental Study on the Link between Optical Emission, Crystal Defects and Photocatalytic Activity of Artist Pigments Based on Zinc Oxide

“…In previous works, we showed how the photoluminescence signal of zinc white changes when the pigment is dispersed in an oil-based binding medium, and we associated these variations with the reactivity of the ZnO surface to adsorb carboxylic acids of the drying oil [20,21]. These outcomes highlighted that the chemical-physical interaction of the pigment with its micro-environment, as well as the metal ion leaching mechanisms, are determined by the presence of crystal defects in the oxide [22]. Inspired by these results, the present research concentrates on the relationships between the crystalline structure and the photocatalytic effect of zinc white from different contemporary manufacturers.…”

A Highly Sensitive and Selective “Off-On” Fluorescent Probe for Cu2+ Based on Rhodamine B Hydrazone

“…It should be noted that Ni 2＋ might form a complex with R1 in spirolactam ring opening form, which is responsible for the UV-vis spectral change, while is non-emissive due to the paramagnetism effect of Ni 2＋ . [35] However, in the case of Cu 2＋ , the intermediate complex between Cu 2＋ and R1 is not stable enough, which occurred a hydrolysis process under the tested condition to produce highly emissive rhodamine B (side infra), according to the fluorescence enhancement. 2＋ and blank.…”